\documentclass{article}
\usepackage{amssymb,amsmath,amscd,concmath}
\pagestyle{empty}
\begin{document}
\begin{equation*}
\rho\frac{Du_i}{Dt}=\rho f_i-\frac{\partial p}{\partial x_i}
+\mu
\left(
  \frac{\partial^2u_i}{\partial x_j\partial x_j}+                                                                                                                                         \frac{1}{3}\frac{\partial\Delta}{\partial x_i}\right)
\end{equation*}
\end{document}
